When rats do a short, intense workout, their muscle-building signals spike a lot right after but fade back to normal within two days.

When rats do the same hard workout again two days later, their muscles don't get the same boost in growth signals as the first time, showing that the body quickly gets used to repeated exercise.

When rats lift weights with different numbers of reps, their muscles show the same early biological changes no matter if they do few or many reps—and those early changes don’t tell us which workout will make muscles grow bigger over time.

When guys who don’t normally lift weights do a tough leg workout on an empty stomach, their muscles show a big spike in two specific signals that tell the body to build more muscle, and this might mean doing more sets makes those signals even stronger.

When you lift weights, your muscles turn on a specific molecular switch called mTORC1, which helps build more muscle protein. But just because this switch flips on right after a workout doesn’t mean you’ll definitely get bigger muscles over time—it doesn’t always line up.

In rats, lifting weights for two weeks made their muscles build more protein even though the usual muscle-building signals in their bodies went down. This shows muscles can grow without needing those normal signals.